Noise reduction structures for electrical machines

ABSTRACT

An electrical machine rotor or stator having a plurality of salient poles and a barrier that inhibits the flow of air along an axial path between the environment outside the rotor or stator and the space between adjacent pairs of the poles. The barrier serves to reduce acoustic noise.

This application claims priority to U.S. provisional application 61/409,638 filed on Nov. 3, 2010, the content of which is incorporated herein by reference.

BACKGROUND OF THE RELATED ART

Switched reluctance machines (SRMs) have made limited entry into commercial applications. A major problem limiting the desirability of using SRMs in commercial applications is the acoustic noise they generate. This acoustic noise is attributed to: (1) high normal forces caused by various imbalances in the non-uniform air gap between an SRM's rotor and stator, (2) discontinuous currents in the SRM's machine windings causing discontinuous torque that produces very high torque pulsations, and (3) the rotor functioning like an impeller. Many emerging applications, such as commercial refrigeration motor drives, require quiet operation with less noise than that of a very good single-speed induction motor. In the case of variable-speed motor drives, the noise of an SRM drive should be comparable to that of a permanent magnet brushless de motor drive. SRMs have not satisfactorily met the noise requirement to satisfy commercial applications.

SUMMARY OF THE INVENTION

The invention disclosed herein provides solutions to the high-noise generation of a switched reluctance machine (SRM) that can be implemented for high-volume applications. High-volume applications require inexpensive machine designs that are simple to implement.

An invention is described in this application with three preferred embodiments for mitigating the acoustic noise of an SRM. Normal forces, imbalance in the air gap, saturation in laminations, and torque ripple all contribute to acoustic noise. The acoustic noise is further exacerbated by electronic switching of current in the SRM's winding.

The rotor of an SRM acts like an impeller and creates fluid flow that is turbulent, which causes significant noise. Also, fringing fluxes flowing from the SRM's stator stack ends to the rotor ends create forces that produce an imbalance in the air gap and eccentricity of the rotor, thereby contributing noise. An object of the invention is to reduce the fluid flow and air gap imbalance by: (1) encapsulating a machine's rotor/stator slots, (2) rotating a machine's stacked rotor laminations, and (3) providing discs on both ends of a machine's rotor/stator stack.

These and other objects of the invention may be achieved, in whole or in part, by an electrical machine component having: (1) a plurality of salient poles, projecting along a radial axis of the component, that each conveys an applied electromagnetic flux; and (2) an electrically and magnetically inert solid material within a space between a rotationally-adjacent pair of salient poles.

Additionally, the objects of the invention may be achieved, in whole or in part, by a machine rotor having: (1) a plurality of salient poles, projecting along a radial axis of the rotor, that each conveys an applied electromagnetic flux; (2) a space between a rotationally-adjacent pair of salient poles that inhibits conveyance of an applied electromagnetic flux; and (3) first and second opposing structures that each extends at least partially across the rotationally-adjacent pair of poles and has an outward salient projection along the radial axis of the rotor.

Still further, the objects of the invention may be achieved, in whole or in part, by an electrical machine having: (1) an electrical component having a plurality of salient poles, projecting along a radial axis of the component, that each conveys an applied electromagnetic flux; and (2) an annulus disposed outside a first axial surface of the electrical component. The annulus is a barrier between a space outside the axial surface of the electrical component and a space between a rotationally-adjacent pair of poles.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in the following paragraphs of the specification and may be better understood when read in conjunction with the attached drawings, in which:

FIGS. 1( a) and 1(b) illustrate a rotor stack assembly having a plurality of laminations;

FIGS. 2( a) and 2(b) illustrate a rotor having its slots filled with a material;

FIG. 3 illustrates a stator having its slots filled with a material;

FIGS. 4( a) and 4(b) illustrate a rotor lamination stack in which a lamination at each end of the stack is phase-rotated with respect to the laminations sandwiched between the two end laminations;

FIG. 5( a) illustrates a rotor lamination stack having phase-shifted end laminations that do not entirely cover the longitudinal sides of rotor slots;

FIG. 5( b) illustrates a rotor lamination stack in which multiple phase-shifted end laminations cover the longitudinal sides of rotor slots;

FIGS. 6( a) and 6(b) illustrate an annulus that blocks the air flow along the axial path of a rotor lamination stack;

FIG. 7( a) illustrates a stator with a plurality of poles and a winding around each pole; and

FIG. 7( b) illustrates an annulus that is disposed at one axial end of a stator so as to cover the open space between each rotationally-adjacent pair of poles.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIGS. 1( a) and 1(b) illustrate a rotor lamination stack 1 having a plurality of laminations. Individual rotor laminations 2(1)-2(x) are stacked together such that their pole faces 11 and slots 12 line up along an axial direction of rotor lamination stack 1. Rotor laminations 2(1)-2(x) are stacked one on top of the other to a length known as a stack length, which determines the torque and power output of a machine comprising rotor lamination stack 1 and a stator (not shown). The stator may be any type of switched reluctance machine (SRM), such as those described by Ramu, Krishnan, “Switched reluctance motor drives”, CRC Press, 2001, which is incorporated herein in its entirety by reference.

SRM rotors have no windings or permanent magnets in their slots 12. Rotor slots 12 are empty spaces between two adjacent teeth 11, otherwise known as rotor poles 11. Rotor poles 11 act as fan blades creating an aerodynamic effect and acoustic noise in the machine, and rotor slots 12 act as air ducts. The fan-blade noise produced by rotor poles 11 has both vortex and tone-blade frequencies. The frequency, f_(rt), of a pure tone arising from air passing through rotor slots 12 may be expressed by:

${f_{rt} = {P_{r} \cdot \frac{N}{60}}},{Hz}$

where P_(r) is the number of rotor poles 11 and N is the rotor speed in revolutions per minute (rpm). The vortex noise component has a very low frequency compared to the tonal-component frequency and is not a major cause for concern in an SRM. The tonal-frequency noise component has exactly the same value as the combined phase frequencies in the SRM. The fan-blade noise component directly adds to the noise component due to normal forces, created by the alignment of the stator and rotor poles, since their frequencies are the same. The noise at this common frequency (i.e., the phase frequency) and its higher-order harmonics, which are integral multiples of this frequency, are the most troubling noise components in the SRM.

An SRM rotor, because of its sizeable slot dimensions compared to the pole face dimensions (e.g., as much as 100 to 125% of the pole face area), provides a large flow surface and area for air circulation as the rotor rotates, creating an aerodynamic effect on the SRM. The aerodynamic effect will be reduced by blocking the airflow path. Such airflow blocking may be achieved by covering the slot volume with a material. Preferably, the covering material is magnetically and electrically inert so that: (1) the flux distribution of the rotor and stator structures is not distorted from the intended design and (2) the material does not create magnetic or electric losses.

Preferably, the material: (1) is sufficiently adhesive to hold on to rotor laminations 2(1)-2(x), (2) has sufficient thermal tolerance to withstand the peak temperature of the rotor, and (3) is inexpensive for high-volume product applications. Such material may be an encapsulation epoxy, resin, or powder.

FIGS. 2( a) and 2(b) illustrate a first embodiment of a rotor 1 having its slots filled with a material. In a preferred embodiment, a filling material 22 of epoxy fills rotor slots 12 and is contoured to be flush with the rotor lamination stack ends and pole faces. If the rotor pole surface is contoured, rather than circular with a constant radius, then material 22 may be contoured to rotor 1's lowest or highest tooth height. Preferably, material 22 is contoured to the lowest rotor pole (i.e., tooth) height so that there is adhesion of the filling material all around the slot without any overhanging that may lead to problems of mechanical retention at high speeds.

Preferably, the technique for filling slots 12 with material 22 is suitable for high-volume production. In a preferred embodiment, rotor lamination stack 1 is placed in a cup having an inner nonstick surface and a height that is flush with rotor lamination stack 1. The cup has a circular surface and its bottom is a disc that can be secured tightly and subsequently removed. The cylindrical periphery of the cup has flexible parts that can be tightened around rotor lamination stack 1 with a latch-like mechanism. After material 22 is cured within rotor lamination stack the stack can be removed by removing the bottom disc from the filling assembly fixture and unlatching the cylindrical part of the cup body. The rotor shaft hole within rotor lamination stack 1 may be masked to prevent poured material 22 from entering. Alternatively, the cup can be made with a protrusion at the center to correspond to the shaft hole of rotor lamination stack 1, so that the placement of rotor lamination stack 1 on the protrusion closes the shaft hole of rotor lamination stack 1. As another alternative, rotor lamination stack 1 is first press fitted to a rotor shaft, placed in a cup surrounding rotor lamination stack 1 to the height of the stack, and then the encapsulating material is poured into the cup. After material 22 is poured or otherwise applied, rotor lamination stack 1 may be cured in a temperature-controlled oven or naturally, by exposing it to air, so that material 22 bonds with rotor lamination stack 1. The bonding is intended to provide good adhesion and mechanical strength for withstanding forces normally encountered in the rotor body.

Curing material 22 in a temperature-controlled oven is quicker and may be accomplished in a few minutes. Curing at ambient temperature may take hours. The appropriate method of curing for a particular application may be chosen based on economic considerations.

Rotor laminations 2(1)-2(x) may be stacked in any manner, such as: (1) symmetrically, with one lamination on top of another so as to provide uniform slots and pole surfaces having no phase shift among them, (2) skewed so as to have a phase shift among laminations, (3) partially skewed, or (4) partial uniform stacking.

The above-described technique of encapsulating space within the slots of a rotor lamination stack may be applied to the slots of a stator or a stator lamination stack. Applying this technique to the stator slots prevents air flow generated by the rotation of the rotor from entering the interstice space, which would create additional friction and noise.

FIG. 3 illustrates a stator having its slots filled with a material. As illustrated, a stator 30 with a plurality of poles 32 and a winding 34 around each pole has a filling material 22 of epoxy. Filling material 22 partially or entirely fills stator slots 36 and is contoured to be flush with the stator lamination stack end-surfaces and pole faces.

Encapsulation with epoxy or another material creates added cost due to the encapsulation material, process, and curing and the fixtures for creating the encapsulation. Particularly for 100 Watt and higher machines, the cost becomes significant, which is undesirable for high-volume applications that are cost-sensitive. Alternatives to encapsulation are described below.

Second Embodiment

FIGS. 4( a) and 4(b) illustrate a second embodiment of a rotor lamination stack 40 in which a lamination at each end of the stack is phase-rotated with respect to the laminations sandwiched between the two end laminations. More specifically, a rotor lamination 42(1) disposed at one longitudinal end of rotor lamination stack 40 is rotated, about a rotational axis of rotor lamination stack 40, so as to be out of phase with rotor laminations 42(2)-42(x−1). From the perspective of a plan view, a pole 11 of rotor lamination 42(1) partially or fully covers a slot 12 of rotor laminations 42(2)-42(x−1). And each slot 12 of rotor laminations 42(2)-42(x−1) is so covered by a pole 11 of rotor lamination 42(1). Similarly, a pole 11 of a rotor lamination 42(x) on the opposite side of rotor lamination stack 40 partially or fully covers a slot 12 of rotor laminations 42(2)-42(x−1) such that each slot 12 of rotor laminations 42(2)-42(x−1) is so covered by a pole 11 of rotor lamination 42(x).

The partial or full covering of slots 12 of rotor laminations 42(2)-42(x−1) inhibits the flow of air into slots 12 along the rotational axis (i.e., longitudinal or axial axis) of rotor lamination stack 40. And when rotor lamination stack 40 is mounted within a stator such that the peripheries of the stator and rotor poles are in close proximity, such close proximity inhibits the flow of air into slots 12 along a radial axis of rotor lamination stack 40. Thus, when rotor lamination stack 40 is mounted within a close-fitting stator, the offset rotor laminations 42(1) and 42(x) and stator pole periphery create a barrier inhibiting the flow of air into and out of slots 12. For structural integrity and mechanical robustness, two or three laminations may be phase rotated at each end of rotor lamination stack 40.

A small increase in eddy current losses exists when laminations of rotor lamination stack 40 are rotated so as to provide a continuous steel surface. The loss can be avoided if the contact points between the phase-rotated laminations and the non-phase-rotated laminations are separated with an electrically and magnetically inert material, such as epoxy.

An advantage of this embodiment is that no special materials or discs have to be made and the use of the rotor laminations to inhibit the flow of air is inexpensive. Additionally, the production process is simple and may be automated to phase shift the end laminations by half a rotor pole pitch from the rest of the lamination stack. Overall, the process for producing this embodiment is easy and inexpensive to implement and the detrimental effects on the performance of the machine are negligible. Experimental results confirm that this embodiment reduces acoustic noise of the machine to an extent equal to that achieved with the first embodiment.

When the arc of rotor pole 11 is greater than the arc of slot 12, rotor laminations 42(1) and 42(x) can be phase rotated to cover an entire slot, as seen from the perspective of a plan view (i.e., as seen along the axis of rotation). When the are of rotor pole 11 is not greater than that of slot 12, a partial covering of slots 12 may be achieved.

FIG. 5( a) illustrates phase-shifted end laminations that do not entirely cover the longitudinal sides of slots 12. More specifically, end laminations 51 and 52 are phase shifted in opposite directions with respect to intermediary laminations 50. As illustrated, no unhindered path exists through a slot 12 along an axis parallel to the axis of rotation. However, end laminations 51 and 52 could be phase rotated in a single direction or opposite directions so as to provide an unhindered path through a slot 12 along an axis parallel to the axis of rotation.

FIG. 5( b) illustrates the use of multiple phase-shifted end laminations for covering the longitudinal sides of slots 12 at one longitudinal end of a rotor lamination stack 54. Within rotor lamination stack 54, a first rotor lamination 55 is phase shifted with respect to intermediary rotor laminations 50 and a second rotor lamination 56 is phase shifted with respect to both first rotor lamination 55 and intermediary rotor laminations 50. Together, first and second rotor laminations 55 and 56 cover all of slots 12 along a longitudinal side of rotor lamination stack 54. Similarly, a pair of laminations 57 and 58 are phase rotated with respect to one another and intermediary rotor laminations 50 so as to cover all of slots 12 along the opposite longitudinal side of rotor lamination stack 54.

Although FIG. 5( b) illustrates that two laminations are phase shifted at each end of rotor lamination stack 54, more than two laminations may be offset with respect to one another and intermediary rotor laminations 50 so as to cover slots 12 along each longitudinal side of rotor lamination stack 54. Covering the entire portion of slots 12 along each longitudinal side provides greater acoustic noise reduction than covering only a portion of slots 12 along each longitudinal side.

Rotor laminations 50 may be stacked in any manner, such as: (1) symmetrically, with one lamination on top of another so as to provide uniform slots and pole surfaces having no phase shift among them, (2) skewed so as to have a phase shift among laminations, (3) partially skewed, or (4) partial uniform stacking. Rotor laminations 51, 52, 55-58 may be identical to intermediary laminations 50, to reduce manufacturing cost, or may have different pole and slot arcs that one another and intermediary laminations 50. Also, the first and second embodiments may be combined so that slots 12 of intermediary laminations 50 are partially or entirely filled with an encapsulating material and bounded by laminations on all sides except an outer radial periphery.

Third Embodiment

FIGS. 6( a) and 6(b) illustrate the use of an annuluses to block air flow along the axial path of a rotor lamination stack 61. Each annulus 62 may be as thick as a rotor lamination but made of lighter and stronger material. An annulus 62 may be placed on each end of rotor lamination stack 61. The annulus material is preferably both electrically and magnetically inert, capable of withstanding the thermal environment of the rotor without any deterioration, and strong enough to withstand the forces surrounding the rotor lamination block. Preferably, each annulus 62 has the same outer diameter as rotor lamination stack 61 or is equal to the minimum diameter of a contoured rotor tooth, so as to have a higher dimensional tolerance.

Thin plastic rings and fiber board used in a printed circuit board base are suitable materials for annulus 62. An advantage of using annulus 62 is that it is easier, in a production environment, to add the annulus than to employ phase-shifted laminations. Also, annuluses are less expensive, lighter in weight, and provide a uniform surface, similar to encapsulation, that is flush with the axial ends of the lamination stack; with phase-shifted laminations, an unevenness of the axial-end surface exists between the slots and poles of the end laminations.

The above-described technique of blocking the air flow along the axial path of a rotor lamination stack may be similarly applied to a stator or a stator lamination stack.

FIGS. 7( a) and 7(b) illustrate the use of an annulus to block the air flow path along the axial path of a stator lamination stack. FIG. 7( a) illustrates a stator 70 with a plurality of poles 72 and a winding 74 around each pole. FIG. 7( b) illustrates, via a plan view, an annulus 76 that is disposed at one axial end of stator 70 so as to cover the open space between each rotationally-adjacent pair of poles 72 without interfering with the rotation of a rotor (not shown) within stator 70. Another annulus (not shown) may be disposed on the opposite axial end of stator 70 in a similar manner.

All three of the above-described embodiments are applicable to any type of electrical machine, including induction motors, permanent magnet synchronous motors, brushless dc motors, and switched reluctance motors. The above-described embodiments may be applied individually or in combination to an electrical machine and to electrical machines having radial or axial field orientations for rotating or linear types of configurations.

Where permanent magnets are buried inside rotor laminations and have flux barriers in the form of closed slots of various sizes and shapes, the flow of air through such slots can cause noise during machine operation. The above-described embodiments can be applied to preventing the flow of air through these slots so as to reduce noise.

The foregoing has been a detailed description of possible embodiments of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Accordingly, it is intended that this specification and its disclosed embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

1. An electrical machine component comprising: a plurality of salient poles, projecting along a radial axis of the component, that each conveys an applied electromagnetic flux; and an electrically and magnetically inert solid material within a space between a rotationally-adjacent pair of salient poles.
 2. The electrical machine component of claim 1, wherein the inert material is epoxy.
 3. The electrical machine component of claim 1, wherein the inert material fills the space between the adjacent pair of poles.
 4. The electrical machine component of claim 1, wherein a distal periphery of the inert material is coextensive with the distal periphery of the adjacent pair of poles.
 5. The electrical machine component of claim 1, wherein the inert material has a length that is coextensive with that of the component along its axis of rotation.
 6. The electrical machine component of claim 1, wherein the component is a rotor of an electrical machine.
 7. The electrical machine component of claim 1, wherein the component is a stator of an electrical machine.
 8. A machine rotor comprising: a plurality of salient poles, projecting along a radial axis of the rotor, that each conveys an applied electromagnetic flux; a space between a rotationally-adjacent pair of salient poles that inhibits conveyance of an applied electromagnetic flux; and first and second opposing structures that each extends at least partially across the rotationally-adjacent pair of poles and has an outward salient projection along the radial axis of the rotor.
 9. The machine rotor of claim 8, further comprising an electrically and magnetically inert solid material within the space bounded by the rotationally-adjacent pair of poles and the first and second opposing structures.
 10. The machine rotor of claim 8, wherein the outer radial-periphery of each of the first and second opposing structures is coextensive with that of the salient poles.
 11. The machine rotor of claim 8, wherein the first and second opposing structures extend entirely across the rotationally-adjacent pair of poles.
 12. The machine rotor of claim 8, wherein: the rotor comprises first laminations that are rotationally aligned so as to form the salient poles and spaces there between, the rotor further comprises second laminations having salient poles and spaces there between that constitute the first and second opposing structures, and the salient poles of the second laminations are rotationally offset from those of the first laminations.
 13. The machine rotor of claim 12, wherein the second laminations each has a smaller pole arc than a pole arc of a first lamination.
 14. The machine rotor of claim 13, wherein a pole of one of the second laminations is offset from all poles of another second lamination.
 15. The machine rotor of claim 13, wherein the poles of each second lamination are offset from those of all other second laminations.
 16. The machine rotor of claim 12, wherein: the second laminations each has a larger pole arc than the pole arcs of the first laminations, and the first and second opposing structures extend entirely across the rotationally-adjacent pair of poles.
 17. The machine rotor of claim 14, wherein: a set of two second laminations are disposed on each end of a stack of first laminations, and for each set of second laminations, a pole of one of the second laminations is offset from all poles of the other second lamination.
 18. The machine rotor of claim 17, wherein the first laminations are identical and the second laminations are identical.
 19. An electrical machine comprising: an electrical component having a plurality of salient poles, projecting along a radial axis of the component, that each conveys an applied electromagnetic flux; and an annulus disposed outside a first axial surface of the electrical component, wherein the annulus is a barrier between a space outside the axial surface of the electrical component and a space between a rotationally-adjacent pair of poles.
 20. The electrical machine of claim 19, wherein the distal periphery of the annulus is coextensive with that of the rotationally-adjacent pair of poles.
 21. The electrical machine of claim 19, wherein the electrical component is a rotor.
 22. The electrical machine of claim 19, wherein the electrical component is a stator.
 23. The electrical machine rotor of claim 19, further comprising an electrically and magnetically inert solid material within the space bounded by the rotationally-adjacent pair of poles and annulus.
 24. The electrical machine of claim 19, further comprising: another annulus disposed outside a second axial surface of the electrical component that is opposite the first axial surface along the axial length of the electrical component, wherein the other annulus is a barrier between a space outside the second axial surface of the electrical component and a space between a rotationally-adjacent pair of poles.
 25. The electrical machine of claim 19, wherein the radial length between the inner and outer radii of the annulus is substantially equal to the radial length of the poles.
 26. The electrical machine of claim 19 further comprising: a stator having a plurality of salient poles, projecting along a radial axis of the stator, that each conveys an applied electromagnetic flux; and another annulus disposed outside a first axial surface of the stator, wherein the electrical component is a rotor.
 27. The electrical machine of claim 19 further comprising: a stator having a plurality of salient poles, projecting along a radial axis of the stator, that each conveys an applied electromagnetic flux; and an electrically and magnetically inert solid material within space bounded by a rotationally-adjacent pair of the stator poles, wherein the electrical component is a rotor.
 28. The electrical machine of claim 19 further comprising: a rotor having a plurality of salient poles, projecting along a radial axis of the rotor, that each conveys an applied electromagnetic flux; and an electrically and magnetically inert solid material within a space bounded by a rotationally-adjacent pair of the rotor poles, wherein the electrical component is a stator.
 29. The electrical machine of claim 19 further comprising: a rotor having a plurality of salient poles, projecting along a radial axis of the rotor, that each conveys an applied electromagnetic flux; a space between a rotationally-adjacent pair of rotor poles that inhibits conveyance of an applied electromagnetic flux; and first and second opposing structures that each extends at least partially across the rotationally-adjacent pair of poles and has an outward salient projection along the radial axis of the rotor, wherein the electrical component is a stator. 